title: Don't birth cohorts matter? A commentary and simulation exercise on Reither, Hauser, and Yang's (2009) age-period-cohort study of obesity.
abstract: Reither, Hauser, and Yang (2009) use a Hierarchical Age-Period-Cohort model (HAPC - Yang & Land, 2006) to assess changes in obesity in the USA population. Their results suggest that there is only a minimal effect of cohorts, and that it is periods which have driven the increase in obesity over time. We use simulations to show that this result may be incorrect. Using simulated data in which it is cohorts, rather than periods, that are responsible for the rise in obesity, we are able to replicate the period-trending results of Reither et al. In this instance, the HAPC model misses the true cohort trend entirely, erroneously finds a period trend, and underestimates the age trend. Reither et al.'s results may be correct, but because age, period and cohort are confounded there is no way to tell. This is typical of age-period-cohort models, and shows the importance of caution when any APC model is used. We finish with a discussion of ways forward for researchers wishing to model age, period and cohort in a robust and non-arbitrary manner.
authors.affiliation: School of Geographical Sciences, University of Bristol, University Road, Bristol, BS8 1SS, UK; Centre for Multilevel Modelling, University of Bristol, 2 Priory Road, Bristol, BS8 1TX, UK. Electronic address: andrew.bell@bristol.ac.uk.
authors.forename: Andrew
authors.initials: A
authors.lastname: Bell
authors.affiliation: School of Geographical Sciences, University of Bristol, University Road, Bristol, BS8 1SS, UK; Centre for Multilevel Modelling, University of Bristol, 2 Priory Road, Bristol, BS8 1TX, UK.
authors.forename: Kelvyn
authors.initials: K
authors.lastname: Jones
mesh_terms: D015988:Cohort Effect; D005260:Female; D054624:Health Status Disparities; D006801:Humans; D008297:Male; D009765:Obesity / Q000453:epidemiology*
keywords: Age–period–cohort models; Collinearity; Model identification; Obesity